The invention relates to audio reception and decoding equipment, and in particular to digital television or radio apparatus for use in the home, in which the loudness levels of audio signals received in different audio formats is equalised for user playback.
Commercial and state owned broadcasting corporations have transmitted audio and video signals for entertainment and information purposes, since the early years of the twentieth century, using analogue encoding techniques, in which a property of the continuously varying electromagnetic wave was used to represent the audio or video information.
In recent years, signals have become transmitted for the most part entirely in a digital format, and there has been a proliferation of equipment available for audio (and video) reproduction in the home. Common devices now include not just television and radio receivers, but integrated receiver devices, such as television receivers, set top boxes, home theatre equipment, stereo equipment and speakers, games consoles, computers, DVD and Blu-Ray™ players. Furthermore, methods of transmitting signals to the home now include more than the traditional over the air or cable broadcasts and include satellite transmissions, copper and fibre optic cable television services, and the internet.
Digital transmission schemes encode the audio information into a digital format optimised for efficient and reliable transmission, rather than for absolute signal fidelity. At the broadcaster, the raw conversion of analogue signals to digital via an analogue to digital converter (ADC) will typically generate digital samples in Linear Pulse Code Modulation format (LPCM). LPCM encodes the analogue audio data as a string of samples, where each sample consists of a number of bits, zeros and ones in digital notation, representing the amplitude of the audio signal at the discrete time instant of that sample.
The LPCM samples are then encoded into one of many available data-reduced (and hence lossy) audio formats for digital broadcast. Lossy audio formats re-encode an approximation of the LPCM audio data in various ways that efficiently represent the time varying frequency and amplitude components of the audio signal. The lossy formats must subsequently be decoded to LPCM signals in a device within the consumer's home, the resulting LPCM samples fed to a digital to analogue converter (DAC), and the analogue signal from DAC fed to an amplifier and speaker in order to be audible to the consumer.
The advantage of this process to broadcasters is that the data-reduced format requires far less frequency spectrum for broadcast than the LPCM format, thus reducing costs and allowing a greater number of channels to be broadcast within a given amount of frequency spectrum.
However, the proliferation of different available audio formats for digital encoding, audio equipment for decoding a received signal, and even standards and protocols for connecting different pieces of equipment together, means that there are a large number of audio pathways an audio signal can take before it is decoded and heard by a user. Each pathway is essentially a combination of audio equipment, codecs and connections. Further, end users who purchase different decoding and playback equipment, may also use that equipment in different ways in accordance with their preferences. Significantly, according to the pathway taken by the audio signal, the processing of the audio signal will be different. Differences will arise from the operation of the encoding and decoding software, as well as the way in which individual pieces of audio equipment output the audio signal.
Depending on the equipment available to the end listener, and the way in which they have configured it for audio reproduction, it is possible therefore that audio signals received from different sources and encoded using different digital encoding formats will once decoded, be heard by the user at different perceived loudnesses. This is undesirable as it means that the listener must frequently search for the remote control and readjust the volume to a comfortable level when changing channels or audio inputs. The end listener is often not aware that the problem arises due to the complexity of the broadcast and programming chain and will believe that the loudness fault lies with the provider of the audio equipment, for example.
We have therefore appreciated that there is a need to address differences in loudness between different audio signals received by the user via different audio pathways.